In the field of bioprosthetic devices, a wide variety of different aortic valve prostheses have been shown in the prior art. Two main categories of valve prostheses can be defined: mechanical valves, including the so-called "caged ball", "caged disc", and "tilting disc" types; and tissue valves, which have leaflets. Of the various aortic valve prostheses currently known, the mechanical valves tend to be more circumferentially rigid than tissue valves. Tissue valves are typically stented and may be more or less circumferentially rigid, depending upon the rigidity of the stent.
It is believed by the inventors, however, that less rigid bioprosthetic valves, particularly ones of the tissue type, would be preferable in some cases since they more closely simulate a natural aortic valve and would therefore be less likely to create problems in the patient with unfavorable systolic and diastolic turbulence patterns, systolic pressure gradients, or embolic episodes. Further, it is believed that compliant bioprosthetic valves, having qualities more closely matched to natural aortic valves, would tend to have better flow efficiency, superior hydraulic characteristics, and flow patterns that are significantly less trauma-promoting and less likely to produce such undesirable effects as thrombus, atherosclerosis, or hemolysis.
While "leaflet" type bioprosthetic valves (of either the stented and non-stented variety) have been shown to more closely resemble a natural aortic valve in operation, experience has shown that fatigue-related failure of the leaflets can occur. This potential has heightened the need for rigorous stress analysis and testing of bioprosthetic valves. Typically, the development of a bioprosthetic valve involves several iterations of the following steps: (1) fabrication of prototypes in various sizes; (2) in vitro (fluid-mechanical, structural, and fatigue) testing of the prototypes; and (3) refinement and re-fabrication.
Of course, it is important for the conditions of any in vitro testing of bioprosthetic devices to simulate, as closely as possible, the in vivo conditions to which the tested devices will be exposed upon implant in patients. In the case of stented bioprosthetic valves, it is a simple matter to rigidly dispose a valve prosthesis, which is itself circumferentially rigid, along a fluid flow path for the purposes of testing. In conventional practice, a stented valve is fitted into a rigid valve holder and secured in place therein by means of a threaded retaining ring. The entire circumferentially rigid valve and valve holder can then be easily introduced into the flow path of various types of testing equipment.
It has been the inventors' experience, however, that in the case of a non-stented valve, it is substantially more difficult to provide a fixture for introducing the non-stented valve into a flow path during in vitro testing that, while providing support for the valve, does not interfere with the physiological functioning of the valve. In particular, it is believed to be desirable to provide a test fixture for non-stented bioprosthetic valves which does not restrict the circumferential compliance of the valve, so that the effects of the valve's compliant circumferential expansion and contraction of the valve can be observed and monitored during the in vitro testing.
In vitro evaluation of non-stented aortic bioprostheses requires that the valve be mounted in a test chamber that reasonably simulates the human aortic root. The use of a simulated or synthetic aortic root has been proposed in the prior art. Artificial aortic roots have been discussed, for example, in Reul et at., "Optimal Design of Aortic Leaflet Prosthesis", American Society of Civil Engineers, Journal of the Engineering Mechanics Division, v. 104, n. 1, Feb. 1978, pp. 91-117; in Ghista et al., "Optimal Prosthetic Aortic Leaflet Valve: Design Parametric and Longevity Analyses: Development of the Avcothane 51 Leaflet Valve Based on the Optimum Design Analysis", Journal of Biomechanics, 10/5-6, 1977 pp 313-324; and in Lu et al., "Measurement of Turbulence in Aortic Valve Prostheses: An Assessment by Laser Doppler Anemometer", Proceedings of a Symposium at the 14th Annual Meeting of the Association for the Advancement of Medical Instrumentation, Las Vegas, Nev., May 21, 1979, Yoganathan et al., editors. The foregoing Reul et al., Ghista et al., and Lu et al. references are incorporated herein by reference in their entirety.
In developing a simulated aorta for in vitro use, several factors must be considered. First, the aortic valve in its natural state does not have a fixed shape, and can only be described at a given time in the cardiac cycle, such as mid-systole or mid-diastole. Second, the human aorta is anisotropic and expands quite easily at low internal pressure but stiffens at higher pressures to prevent ballooning (this is discussed in Thubrikar et al., "Normal Aortic Valve Function in Dogs", American Journal of Cardiology, vol. 40, October 1977; in Brewer et al., "The Dynamic Aortic Root", Journal of Cardiovascular Surgery, Jun. 3, 1976; and in Ferguson et al., "Assessment of Aortic Pressure-Volume Relationships With an Impedance Catheter", Catheterization and Cardiovascular Diagnosis, 15:27-36, 1988). The foregoing Thubrickar et al., Brewer et al., and Ferguson et al. references incorporated herein by reference in their entirety.
Finally, since in vitro evaluation of an aortic bioprosthesis requires extended testing, a material which provides bacterial stability is necessary. Materials such as rubber provide bacterial stability and can be easily produced to exact geometric dimensions, but these materials are isotropic and do not exhibit the same locking characteristics at high pressures that are seen with anisotropic materials. For these reasons, it would be advantageous to provide a simulated aorta of repeatable geometric design and having controllable compliance characteristics to provide reasonable in vitro model aortas.
With the in vitro testing arrangement proposed by Lu et al. in the above-cited reference, the compliance factor for a flow loop system including a simulated aortic root is provided not by the simulated root itself, but rather by means of a compliance chamber disposed on the outflow side of the valve being tested. In the above-cited Reul et al. and Ghista et al. references, the artificial aortic root is made from polyurethane by a dipping process, so that the desired compliance is achieved by controlling the thickness of the polyurethane at the time the artificial aorta is fabricated. The Reul et al. flow loop additionally contains a compliance element for approximating natural compliance factors during testing.
There are numerous types of tests that must be performed on bioprosthetic aortic valves in order to evaluate their effectiveness and in order to obtain regulatory agency approval for public use of such devices. Among the more common of these tests are: steady flow studies, which focus on hydromechanical performance of the valves; pulsatile flow studies, which are concerned with valve dynamics (opening and closing times, leaflet motion, and the like), forward and backward (regurgitating) flow patterns, the pressure gradients across the valve, and energy loss across the valve; and fatigue studies, which are concerned with the ability of the valve to withstand millions of cycles without fatigue-related failure.
In the prior art, it has generally been the case that all of the necessary in vitro test data cannot be obtained from a single model setup, and that for each different analytical purpose, specially designed test apparatus is required. Depending upon the nature of the data to be obtained from a particular test setup, certain aspects of the in vivo environment may not be accounted for in the setup. For example, it has been suggested in the prior art (see, e.g., the above-cited Reul et al. reference) that for the purposes of steady flow tests, it is sufficient to dispose the valve being tested in a simple straight tube of constant diameter, rather than in a flow channel that simulates the subtle geometries of a human aorta. Likewise, for example, it may be deemed unnecessary during fatigue testing to simulate the circumferential compliance of the aortic root, whereas the compliance factor might be considered highly significant during pulsatile flow analysis.
Since several different test apparatuses may be required to obtain desired test data for a device, and since the different test apparatuses may be commercially-available from different vendors, it is often the case that the fixtures used to support the device being tested in a flow loop will differ from one test setup to the next. Depending upon the type of fixture used for a given setup, it may be difficult or impossible to utilize the same valve and/or simulated aortic root in more than one test. As a result, test data obtained from one test setup may not be accurately and meaningfully correlated with the data obtained from a different test. The inability to re-use a valve in a test setup may even limit the ability to obtain reproducible results even from the same test setup.